As energy consumption continues to increase, the exploitation and utilization of non-conventional energy resources, such as shale gas and shale oil, etc., receives attention gradually. The explored shale oil and gas reservoir in China is huge, and is of great value for exploration and development. At present, a multi-staged fracturing method is mainly used in long horizontal wells for shale gas mining, in order to improve well yield and industrial exploitation value. Owing to the fact that the shale formation has high water sensitivity and developed crevices, complicated situations such as wall collapse and bore shrinkage, etc. may occur in the drilling process. Therefore, the wall stability problem of long horizontal wells is a major technical difficulty encountered in the development of drilling fluids and completion fluids for shale gas mining.
At present, oil-based drilling fluids are mainly employed for horizontal wells in shale formation, and such oil-based drilling fluid can solve the well wall stability problem satisfactorily. However, the well yield of shale gas is usually low, and the cost of oil mud is high, causing compromised industrial value of shale gas mining; in addition, oil-based drilling fluids have problems including poor environmental protection property, high recycling cost, and poor safety, etc. Water-based drilling fluids have advantages including lower cost, more environment friendly, and easy access to the oil and gas reservoir, etc.; however, common water-based drilling fluid systems have poor well wall stability performance and inadequate lubrication and anti jamming capability. Consequently, the application of water-based drilling fluids in shale oil and gas exploitation is limited.
In China, most of the shale gas reservoirs are buried deeply (e.g., the Lower Cambrian Series shale gas formation in Sichuan Basin is in 2,000-3,500 m burial depth), have high formation pressure, and involve complex formations, including super-high pressure hydrocarbon formation, saltwater formation, and argillaceous rock formation, etc. When a shale formation that has high gas content and highly developed crevices is encountered during well drilling, the severely increased gas leakage may result in accidents such as well blowout and well kick, etc. By increasing the density of the drilling fluid, the formation pressure may be balanced effectively, the well wall stability can be improved, and the occurrence of complex situations can be prevented.